1) Field of the Invention
This application relates generally to electronic circuits and more particularly to an electronic circuit for detecting a zero current condition, where such a circuit can be used in voltage regulators and switching power converters (xe2x80x9cSPCxe2x80x9d), including multiphase power converters.
2) Background
Power regulators are often used in electronic equipment to supply power at a predetermined voltage to a system. For example, a typical desktop computer may contain a power supply that converts alternating current (xe2x80x9cACxe2x80x9d) from a wall socket, to direct current (xe2x80x9cDCxe2x80x9d) with a voltage that is usable by the various components of the computer system. With continued reference to computer systems, a hard disk drive may require a 12 volt (xe2x80x9cVxe2x80x9d) power input, while various integrated circuit components may require, for example, power at 5.0 V, 3.3 V, or 1.5 V. A power supply must thus contain power regulators to generate the required voltage levels.
Buck power regulators are often used to generate power outputs for microelectronic devices because they are relatively efficient and provide high current stewing (di/dt) capability. When providing a microprocessor with a regulated input voltage, di/dt and response time are very important considerations. The output inductor value of the regulator determines the di/dt capability of the regulator and also the boundary between continuous conduction mode (xe2x80x9cCCMxe2x80x9d) (when the inductor current is continuous) and discontinuous conduction mode (xe2x80x9cDCMxe2x80x9d) (when the inductor current is not continuous, but drops to zero until the transistor is turned ON; DCM typically occurs when a low load resistance is coupled to the buck power regulator).
With reference to FIG. 1, an exemplary buck (step-down) power regulator 100, which converts a DC voltage to a lower voltage, is presented. A supply voltage, Vs, is input into transistor 102, which is coupled to a diode 104 that, in turn, is coupled to ground. Coupled to the junction of transistor 102 and diode 104 is an LC circuit comprising an inductor 106 and a capacitor 108. A load 110 thus receives power at the required voltage, where the voltage is determined by the duty cycle of transistor 102 (i.e., the percentage of time when transistor 102 is turned on).
When transistor 102 is on, inductor 106 is being charged and the supply voltage supplies the output current. When transistor 102 is turned off, inductor 106 xe2x80x9cfreewheelsxe2x80x9d through diode 104 and supplies the energy to load 110. The purpose of the diode is not to rectify, but to re-direct current flow in the circuit and to ensure that there is a path for the current from the inductor to flow. Capacitor 108 serves to reduce the ripple content in the voltage, while inductor 106 smoothes the current passing through it.
A problem of the buck power regulator is that, as low voltage outputs are required, the voltage drop of diode 104 leads to various consequences. For example, the circuit becomes less efficient because of the voltage drop of approximately 0.7 volt across the diode. Such inefficiencies become less tolerable when devices run on battery power as opposed to AC power.
In response to the above deficiencies, buck power regulator 200, detailed in FIG. 2, was developed. As can be seen, buck power regulator 200 is similar to buck power regulator 100, with a transistor 204 replacing diode 104. Transistor 204 may be configured to have a low on resistance. Transistor 102 is usually termed the high-side switch and transistor 204 is the low-side switch. In addition drivers 222 and 224 control the operation of transistors 102 and 104, respectively. By controlling the on and off cycles of transistors 102 and 204, drivers 222 and 224 are able to more efficiently control the output voltage, Vout, that is present at load 110, and supply the desired amount of current.
In normal operation of a power converter, there is a ripple in the output current, due to the charging and discharging of inductor 106. One method of reducing the ripple of the output current is the use of a multiphase power supply. Instead of having, for example, a single source supplying a 20 amp output, there may be four phases, each of which supply 5 amps. An exemplary multiphase buck power converter is shown in FIG. 12.
In multiphase power converter 1200, it is desired to convert an input voltage 1202 to an output voltage 1204 across a load 1206. In a manner similar to that described above with respect to FIG. 2, transistors 1212 and 1214 are each coupled to the input voltage 1202. Coupled to the junction 1211 of transistors 1212 and 1214 is inductor 1216. Similarly, transistors 1222 and 1224 are each coupled to the input voltage 1202. Coupled to the junction 1221 of transistors 1222 and 1224 is inductor 1226. Similarly, transistors 1232 and 1234 are each coupled to the input voltage 1202. Coupled to the junction 1231 of transistors 1232 and 1234 is inductor 1236. Similarly, transistors 1242 and 1244 are each coupled to the input voltage 1202. Coupled to the junction 1241 of transistors 1242 and 1244 is inductor 1246. Each of the transistor pairs is coupled to capacitor 1208 to provide the output needed at output 1204. Because of the presence of four power converters, each converter is only responsible for one-fourth of the total current needed, resulting in smaller transistors and inductors and a corresponding reduction in cost. In addition, the ripple in the output current is reduced because each of the converters is only responsible for a portion of the output current. The phases are slightly offset from each other such that the peak current of each individual phase do not coincide with each other. This is shown in FIG. 15, which shows the individual output currents for each phase as well as the total output current. As can be readily seen, the ripple in the output current is substantially reduced from the ripple in the current of each individual phase, and the period of the ripple is approximately one-fourth of the ripple of each individual phase.
FIG. 3 presents a plot of the inductor current of an exemplary buck power regulator. Axis 302 represents the passage of time, while axis 304 details the current flowing through inductor 106. The current flowing through inductor 106 rises for the time period Ton when transistor 102 is on and the current falls during time period Toff, when transistor 102 is off. The period, T, is Ton plus Toff. The output voltage would be the input voltage times Ton.
Problems may arise, however, when buck power regulator 200 is required to produce a voltage through a smaller load. An exemplary resulting current plot is shown in FIG. 4. It can be seen that the current through inductor 106 becomes negative during a portion of the cycle, i.e., the current through inductor 106 reverses direction and flows into the ground. This behavior is undesirable because of the various inefficiencies that occur because the inductor is basically wasting power that would ideally remain in the system. Such a problem may not be present in buck power regulator 100 of FIG. 1, as diode 104 automatically xe2x80x9cturns offxe2x80x9d when the polarity of the inductor current changes.
It is desirable to develop a method and apparatus for converting voltage that alleviate the above and other problems that may be present in the prior art.
The present invention uses a Zero Current Detection (xe2x80x9cZCDxe2x80x9d) circuit to determine the direction of current flow in various circuits, such as a switch of a switching power converter (xe2x80x9cSPCxe2x80x9d). In such a manner, once zero current is detected, the operation of the circuit can be changed such that inefficiencies are reduced.
In one embodiment, the ZCD circuit may comprise a pair of current mirrors supplying current to a matched pair of transistors. One of the transistors is coupled to ground while the other transistor is coupled to the node of interest. The outputs of the matched pair are input into a comparator. When the non-inverting input voltage exceeds the inverting input voltage, the comparator changes state.
In one embodiment, the ZCD circuit may be used in a SPC that is configured as a buck converter having Field Effect Transistors (xe2x80x9cFETsxe2x80x9d) used as power switches. The ZCD signal may be used to maximize the efficiency of the system by controlling the operation of the FETs during DCM operation. In such a manner, the current flow through the inductor is prevented from becoming negative.
In another embodiment, the ZCD circuit may be used in a multiphase power converter in a tri-state mode to decrease the switching time when transients occur.
The result is increased system efficiency and faster transient response.